# Second cohomology group for trivial group action of D16 on Z2

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This article gives information about the second cohomology group for trivial group action (i.e., the second cohomology group with trivial action) of the group dihedral group:D16 on cyclic group:Z2. The elements of this classify the group extensions with cyclic group:Z2 in the center and dihedral group:D16 the corresponding quotient group. Specifically, these are precisely the central extensions with the given base group and acting group.
The value of this cohomology group is elementary abelian group:E8.
Get more specific information about dihedral group:D16 |Get more specific information about cyclic group:Z2|View other constructions whose value is elementary abelian group:E8

## GAP implementation

### Construction of the cohomology group

The cohomology group can be constucted using the GAP functions CyclicGroup, DirectProduct, TwoCohomology, TrivialGModule, GF.

```gap> G := DihedralGroup(16);;
gap> A := TrivialGModule(G,GF(2));;
gap> T := TwoCohomology(G,A);
rec( group := <pc group of size 16 with 4 generators>,
module := rec( field := GF(2), isMTXModule := true, dimension := 1,
generators := [ <an immutable 1x1 matrix over GF2>,
<an immutable 1x1 matrix over GF2>,
<an immutable 1x1 matrix over GF2>,
<an immutable 1x1 matrix over GF2> ] ),
collector := rec( relators := [ [ 0 ], [ [ 2, 1, 3, 1, 4, 1 ], [ 3, 1 ] ],
[ [ 3, 1, 4, 1 ], [ 3, 1 ], [ 4, 1 ] ],
[ [ 4, 1 ], [ 4, 1 ], [ 4, 1 ], 0 ] ], orders := [ 2, 2, 2, 2 ],
wstack := [ [ 1, 1 ], [ 3, 1, 4, 1 ], [ 4, 1 ] ], estack := [  ],
pstack := [ 3, 5, 3 ], cstack := [ 1, 1, 1 ], mstack := [ 0, 0, 0 ],
list := [ 0, 0, 0, 0 ],
module := [ <an immutable 1x1 matrix over GF2>,
<an immutable 1x1 matrix over GF2>,
<an immutable 1x1 matrix over GF2>,
<an immutable 1x1 matrix over GF2> ],
mone := <an immutable 1x1 matrix over GF2>,
mzero := <an immutable 1x1 matrix over GF2>, avoid := [  ],
unavoidable := [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ] ),
cohom := <linear mapping by matrix, <vector space of dimension 5 over GF(
2)> -> ( GF(2)^3 )>,
presentation := rec( group := <free group on the generators
[ f1, f2, f3, f4 ]>,
relators := [ f1^2, f1^-1*f2*f1*f4^-1*f3^-1*f2^-1, f2^2*f3^-1,
f1^-1*f3*f1*f4^-1*f3^-1, f2^-1*f3*f2*f3^-1, f3^2*f4^-1,
f1^-1*f4*f1*f4^-1, f2^-1*f4*f2*f4^-1, f3^-1*f4*f3*f4^-1, f4^2 ] ) )```

### Construction of extensions

The extensions can be constructed using the additional command Extensions.

```gap> G := DihedralGroup(16);;
gap> A := TrivialGModule(G,GF(2));;
gap> L := Extensions(G,A);;
gap> List(L,IdGroup);
[ [ 32, 39 ], [ 32, 18 ], [ 32, 19 ], [ 32, 9 ], [ 32, 14 ], [ 32, 20 ],
[ 32, 19 ], [ 32, 9 ] ]```

### Under the action of the various automorphism groups

This uses additionally the GAP functions AutomorphismGroup, DirectProduct, CompatiblePairs, and ExtensionRepresentatives.

```gap> G := DihedralGroup(16);;
gap> A := TrivialGModule(G,GF(2));;
gap> A1 := AutomorphismGroup(G);;
gap> A2 := GL(1,2);;
gap> D := DirectProduct(A1,A2);;
gap> P := CompatiblePairs(G,A,D);;
gap> M := ExtensionRepresentatives(G,A,P);;
gap> List(M,IdGroup);
[ [ 32, 39 ], [ 32, 19 ], [ 32, 9 ], [ 32, 18 ], [ 32, 14 ], [ 32, 20 ] ]```